The present study is part of a long-term comparative investigation testing the hypothesis that the striatal complex plays a basic role in organizing the expression of species-typical, prosematic ("nonverbal") behavior. In mammals, the striatal complex includes the olfactostriatum, corpus striatum (caudate and putamen), globus pallidus, and satellite collections of gray matter. In preceding experiments it was found that lesions of the medial segment of the globus pallidus or interruption of its projections to the ventral thalamus or to the tegmentum affects the expression of the species-typical mirror display of gothic-type squirrel monkeys. Recovery of the display may occur after extensive damage to the thalamic division, but not the tegmental division. The purpose of the present study is to learn which of the tegmental projections is most essential for the expression of the display.